How do I make my camera in my main scene move using buttons in an overlay UI scene?
GUI

Get rid of the "moving" logic (not needed with the below method)

Replace all property actuators with message actuators. Choose meaningful subject names, such as "move forward". 

Enable True Level Triggering at the according sensors to send the messages as long as the operation is requested (-> the key is pressed).

Camera

Replace all property sensors with message sensors e.g. listening on "move forward". 

Remarks:


Interobject connections are not needed. 
You can have as many object serving as UI as you want
UI objects can reside in any active scene.


I hope it helps